This project is aimed at characterizing the mechanisms by which a rhodopsin molecule excites the visual receptor. There are four major objectives: (1) By observing the rate at which rhodopsin undergoes rotational diffusion in the membrane, we will attempt to detect the time-course of any direct interactions a photoactivated rhodopsin molecule (metaII) has with the three enzymes it activates (GTPase, phosphodiesterase, kinase). (2) Structural characteristics of rhodopsin will be obtained by comparing flash-photometric observations of rhodopsin with those from bacteriorhodopsin whose structure is now the best determined of all membrane proteins: both proteins will be incorporated into the same type of lipid vesicles to enable sensitive and direct comparisons of the chromophore angles and motions, the effective radius of the two proteins for rotational diffusion in the membrane, and the relative exposures of the two proteins to the viscosity of the aqueous phase. (3) The role played by the fluid nature of the disc membrane will be investigated by altering the viscosity of the membrane and observing the effects on different stages in the excitation process. (4) The role of diffusional processes in visual excitation and adaptation will be investigated theoretically by extending and developing the "diffusion" model of visual excitation. The role of diffusion in cell organelles similar to the outer segment will also be examined with the aim being to identify which, if any, of the dimensions and structural relationships are determined by diffusion-limited delay times. Two questions which will be pursued are: Why does the speed of the receptor potential correlate so well with the diameter of the outer segment? - - By what mechanism might the receptor cell "detect" the length of its outer segment, and thereby be able to regulate the length? Experimental techniques to be employed include flash photometry, EPR spin probes of membrane viscosity, electrophysiology of the retina, lipid and protein chemistry.